Abstract
The effects of chemical size and valence on the crystal structure, magnetoresistance and magnetic properties were investigated in a perovskite-like (La 0.7 Ba 0.3 )(Mn 1-x Co x )O 3 system with x = 0-1.0. The samples with x≤0.3 and x≥0.9 are of a cubic phase, but an orthorhombic phase is observed in the samples with 0.4≤x≤0.8. The lattice parameter, a, based on the cubic phase, at first decreases with increasing x up to 0.7, then slightly varies as x≥0.8. The metal-insulator transition shows up in the compositions for x≤0.2, and subsequently disappears as x≥0.3. All samples exhibit ferromagnetism at low temperatures, but the long-range magnetic ordering in pure manganate (x = 0) becomes a cluster glass-type (short-range) ferromagnetic order due to the introduction of Co. The saturation magnetization decreases with increasing Co content for x≤0.7, then increases as x≥0.7. Our recent study using an X-ray absorption spectroscope has revealed that Co 2+ /Co 3+ and Mn 3+ /Mn 4+ , but no Co 4+ or Mn 2+ ions were observed in this entire series. The magnetotransport and magnetic properties are closely related to the average valences of Mn and Co ions.